1. Field of the Invention
The present invention relates to a power supply device including an auxiliary power supply unit and an image forming apparatus including the power supply device.
2. Description of the Related Art
In general, a typical power supply device used in an image forming apparatus supplies an electric power from a commercial power source to each load by converting an alternating-current (AC) voltage of the electric power into a direct-current (DC) voltage in a rectifier/smoothing circuit and controlling a predetermined constant voltage of the electric power to be supplied. When a heavy load is required, for example, for making copies, the electric power supplied from only the commercial power source may not be enough. Therefore, the image forming apparatus usually includes an auxiliary power supply unit composed of a capacitor. The auxiliary power supply unit is preliminarily charged with an electric power from the commercial power source. When a power shortage occurs or is anticipated, the auxiliary power supply unit supplies the charged electric power to each load by discharging the electric power via a discharge circuit (see, for example, Japanese Patent Application Laid-open Publication No. 2004-286869).
The load usually requires a high voltage and a high power, so that it is preferable that the capacitor of the auxiliary power supply unit has a high withstand voltage and a high charge capacity. However, a typical capacitor has a low withstand voltage, so that a plurality of capacitor cells needs to be connected in series with one another to enable the capacitor to withstand a high voltage. Such a capacitor is very expensive because a large number of the capacitor cells are required. Therefore, the number of the capacitor cells is reduced, and a DC/DC converter (a voltage converting unit) is provided to the discharge circuit located in a subsequent stage of the capacitor. An electric power discharged from the capacitor is boosted by the DC/DC converter, and the boosted electric power is supplied to each load.
However, with the above configuration, when the power supply device supplies an electric power to a load in which an inrush current (a starting current) is generated, and the load starts being driven, a high current from the capacitor is temporarily flown into the a load located in a subsequent stage of the DC/DC converter through the DC/DC converter. Due to the high current, a power loss occurs at a coil of the DC/DC converter, and thereby decreasing an efficiency of the DC/DC converter. In addition, the DC/DC converter needs to be the one with an element capable of withstanding the high current, so that a cost of the power supply device increases.